1. Field of Invention
The present invention relates in general to the field of electronic packaging, and in particular to electronic packaging that removes heat from an electronic component.
2. Background Art
Electronic components, such a microprocessors and integrated circuits, must operate within certain specified temperature ranges to perform efficiently. Excessive heat degrades electronic component performance, reliability, life expectancy, and can even cause failure. Heat sinks are widely used for controlling excessive heat. Typically, heat sinks are formed with fins, pins or other similar structures to increase the surface area of the heat sink and thereby enhance heat dissipation as air passes over the heat sink. In addition, it is not uncommon for heat sinks to contain high performance structures, such as vapor chambers and/or heat pipes, to further enhance heat transfer. Heat sinks are typically formed of metals, such as copper or aluminum. More recently, graphite-based materials have been used for heat sinks because such materials offer several advantages, such as improved thermal conductivity and reduced weight.
Electronic components are generally packaged using electronic packages (i.e., modules) that include a module substrate to which the electronic component is electronically connected. In some cases, the module includes a cap (i.e., a capped module) which seals the electronic component within the module. In other cases, the module does not include a cap (i.e., a bare die module).
Bare die modules are generally preferred over capped modules from a thermal performance perspective. In the case of a capped module, a heat sink is typically attached with a thermal interface between a bottom surface of the heat sink and a top surface of the cap, and another thermal interface between a bottom surface of the cap and a top surface of the electronic component. In the case of a bare die module, a heat sink is typically attached with a thermal interface between a bottom surface of the heat sink and a top surface of the electronic component. Bare die modules typically exhibit better thermal performance than capped modules because bare die modules eliminate two sources of thermal resistance present in capped modules, i.e., the thermal resistance of the cap and the thermal resistance of the thermal interface between the cap and the electronic component. Accordingly, bare die modules are typically used to package electronic components that require high total power dissipation.
Heat sinks are attached to modules using a variety of attachment mechanisms, such as clamps, screws, and other hardware. The attachment mechanism typically applies a force that maintains a thermal interface gap, i.e., the thickness of the thermal interface extending between the heat sink and the module. In the case of a capped module, the cap protects the electronic component from physical damage from the applied force. In the case of a bare die module, however, the applied force is transferred directly through the electronic component itself. Consequently, when bare die modules are used, the attachment mechanism typically applies a compliant force to decrease stresses on the electronic component.
FIG. 1 illustrates an example of a prior art attachment mechanism for attaching a heat sink to a bare die module using a compliant force. A circuit board assembly 100 includes a printed circuit board 105 and a bare die module 110. Bare die module 110 includes a module substrate 115, an electronic component such as a semiconductor chip 120, and an electronic connection 125. Semiconductor chip 120 is electrically connected to module substrate 115. Electronic connection 125, which electrically connects printed circuit board 105 to module substrate 115, may be a pin grid array (PGA), a ceramic column grid array (CCGA), a land grid array (LGA), or the like. Semiconductor chip 120 is thermally connected with a heat sink 130 through a thermal interface 135, which is a layer of thermally conductive material such as thermal paste, oil, or other high thermal conductivity material. Typically, the thermal interface 135 is relatively thin so that it may easily transfer heat away from bare die module 110 and toward heat sink 130. The thickness of thermal interface 135 extending between heat sink 130 and semiconductor chip 120 is referred to as the thermal interface gap.
Heat sink 130 is attached to bare die module 110 using bolts 140. Bolts 140 pass through thru-holes 131 in heat sink 130 and thru-holes 106 in printed circuit board 105 and are threaded into threaded-holes 146 in a backside bolster 145. Typically, bolts 140 are arranged one at each corner of the electronic component 120, or one on each side of the electronic component 120. Bolts 140 are tightened by threading a threaded portion of bolts 140 into threaded-holes 146 in backside bolster 145. As bolts 140 are tightened, heat sink 130 engages semiconductor chip 120 through thermal interface 135. Additional tightening of bolts 140 causes deflection of printed circuit board 105 which applies a compliant force to bare die module 110. More particularly, printed circuit board 105 is slightly flexed in a concave-arc fashion with respect to bare die module 110.
Unfortunately, deflection of printed circuit board 105 will not always provide the necessary compliance to decrease stresses on electronic component 120. Problems may arise with this board deflection approach if, for example, printed circuit board 105 has a relatively thick cross-section and/or bare die module 110 has a relatively large area (i.e., the “footprint” occupied by bare die module 110 on printed circuit board 105). Printed circuit boards of relatively thick cross-section are typically less compliant than printed circuit boards of thinner cross-section. Consequently, the necessary compliance often cannot be achieved by deflecting relatively thick cross-section printed circuit boards. Moreover, the resulting stresses on the printed circuit board upon deflection can lead to catastrophic failure of solder joints and conductor traces on the printed circuit board. In addition, if the bare die module has a relatively large area, the concave-arc that results upon deflection of the printed circuit board can put the solder joints of the bare die module under unacceptable tension stresses.